1. Field of the Invention
This invention relates to an apparatus and a method for measuring vibrations and in particular to such apparatus and method for measuring torsional vibrations of a rotating shaft.
2. Description of the Prior Art
In large machines having a rotating shaft which may weigh several tons, very small torsional vibrations in the shaft will cause stresses which may damage the shaft and in some instances result in shaft breakage and destruction of the machine. Such vibrations may be of magnitudes as small as 0.01.degree.. A device capable of detecting and measuring such small vibrations has been disclosed in U.S. Pat. No. 3,885,420.
An embodiment of the measuring apparatus disclosed in the above-mentioned patent comprises a gear adapted for rotation with a shaft, a magnetic probe for producing an electrical signal in response to movement of the gear teeth past the probe and a detection circuit. The signal is applied to the detection circuit which produces first and second pulse trains representing the instantaneous and average angular velocities of the shaft, respectively. The detection circuit also compares the phase of each pulse in the first pulse train to the phase of a corresponding pulse in the second pulse train. Any instantaneous change in the angular position of the shaft from a uniformly increasing value will result in a change in the phase relationship between the corresponding pulses and the detection circuit will produce an output signal representing the amplitude and frequency of torsional vibrations in the shaft.
Although the apparatus disclosed in the above-mentioned patent is highly sensitive to small torsional vibrations, it is not usable for the measurement of vibrations at certain frequencies. Specifically, it cannot be utilized to measure vibrations occurring at frequencies which are multiples of the cyclic rate of rotation of the shaft (shaft speed). At these frequencies, the output signal includes false components which cannot be distinguished from true components representing actual shaft vibrations. These false components are caused by inhomogeneities in the gear in the form of irregular spacing between the teeth or differences in the metallurgical compositions of the teeth.
It is the primary object of the present invention to provide torsional vibration measuring apparatus of the above-described type which has the additional capability of unambiguously measuring such vibrations occurring at frequencies which are multiples of the shaft speed.